1. Field of the Invention
The present invention generally relates to belt driving control apparatuses and image forming apparatuses, and more specifically to a belt driving apparatus configured to control driving of an endless belt such as a transferring conveyance belt used for a transferring apparatus or the like of a color image forming apparatus, and an image forming apparatus, such as a color printer or color copier, having the belt driving control apparatus.
2. Description of the Related Art
There are two methods, a direct transferring method and an intermediate transferring method, as general methods for forming a color image in an image forming apparatus. In the direct transferring method, toner images having different colors and formed on plural photosensitive bodies are directly superposed (overlapped) on a transferring paper so that transferring is made. In the intermediate transferring method, the toner images having different colors are transferred to an intermediate transferring body and then transferred to the transferring paper in a lump (superposed). These methods are called tandem methods because plural photo sensitive bodies are commonly arranged in line so as to face the transferring paper or the intermediate transferring body in these methods. An electrophotographic process such as forming an electrostatic latent image or developing is implemented to for respective yellow (Y), magenta (M), cyan (C) and black (K) colors for the photosensitive body. In the direct transferring method, transferring is made onto the running transferring paper. In the intermediate transferring method, transferring is made onto the running intermediate transferring body.
It is general practice, in a tandem type color image forming apparatus that an endless belt, which runs while carrying the transferring paper, be used in the direct transferring method, and an endless belt which receives and carries the image from the photosensitive body be used in the intermediate transferring method. An image forming unit including four photosensitive bodies is arranged in line along a running side of the endless belt.
In the above-discussed tandem type color image forming apparatus, it is important to securely stack (superpose) toner images of respective colors so that generation of a color registration is prevented. Because of this, in order to prevent such a color registration due to the speed variation of the transferring belt, in either transferring method, an encoder is provided at one of plural dependent shafts forming the transferring unit and the rotational speed of the driving roller is feed-back controlled as corresponding to the rotational speed variation of the encoder.
As a most general method for realizing such a feed back control, a proportional control (PI control) is used. In this control, first, a position deviation e(n) is computed from a difference between an object angular displacement Ref(n) of the encoder and the detection angular displacement P(n-1) of the encoder. Then, a low-pass filter is applied to the position deviation e(n) that is a computed result so that high frequency noise is eliminated, a control gain is applied, and a certain standard driving pulse frequency is added. By controlling the driving motor by using the obtained driving pulse frequency, it is possible to control the encoder output so that the encoder output is driven at an object angle deformation.
In actual control, a counter for counting a starting edge of an output of the encoder pulse and a counter for counting a control cycle such as 1 ms are used. The position deviation can be obtained based on the difference between a computing result of the object angle deformation moving during a control cycle (1 ms) and a detection angle deformation obtained by obtaining the encoder count value every the control cycle.
More specifically, the following computing is implemented under a condition that the roller diameter of a dependent shaft where the encoder is attached is 15.615 mm.e(n)=θ0×q−θ1×ne [rad]
Here, e(n)[rad] represents a position deviation computed by sampling this time. θ0[rad] represents a moving angle per a control cycle and is equal to 2π×V×10−3/15.615π[rad]. θ1[rad] represents a moving angle per encoder 1 pulse and is equal to 2π/p[rad] wherein p represents a slit pitch of the encoder. “q” represents a count value of a control cycle timer. “ne” represents an encoder count value. “V” represents a belt linear speed [mm/s].
For example, 300 pulses per one rotation are used as a resolution of the encoder at a control cycle of 1 ms. The feed back control is applied so that the transferring belt is moved at 162 mm/s. As a result, θ0 and θ1 are calculated as follows.θ0=2π×162×10−3/15.615π=0.0207487[rad]θ1=2π×p=2π/300=0.0209439[rad]
The above-mentioned computing is performed every control cycle so that the position deviation is obtained and the feed back control is implemented.
However, in this method, the conveyance speed of the transferring paper is changed due to the minute thickness of the conveyance belt so that the image is shifted from the ideal position and the image quality is degraded. Furthermore, the image is changed between plural recording papers so that the repeated reproducibility of the image forming position between the recording papers is degraded.
Assuming that the conveyance speed is determined in the center of the belt thickness, the belt conveyance speed V is calculated as follows.V=(R+B/2)×ω
Here, R represents a driving roller radius, B represents a belt thickness, and ω is an angular speed of the driving roller.
Meanwhile, FIG. 1 is a view showing the relationship between a belt thickness B of the conveyance belt and a belt driving effective radius r for explaining problems of the related art. As shown in FIG. 1, as the belt thickness B of the conveyance belt 50 is changed, a position of an effective line of the belt thickness shown by a dotted line is changed. This means, the belt driving effective radius “r” is changed. Since “R+B/2” is changed, even if the angular speed ω of the driving roller 51 is constant, the belt conveyance speed is changed. In other words, even if the driving roller 51 is rotated so that the anglar speed is constant, as long as the belt thickness is changed, the belt conveyance speed is changed.
FIG. 2 is a view showing a model of a driving conveyance system of the conveyance belt 50 wound around a driving roller 51, an idler roller 52, and a tension roller 53. FIG. 3 is a graph showing a belt thickness displacement and a belt speed variation in a single circle of the conveyance belt 50 in a case where the driving roller 51 is rotated at a certain angular speed. In a case where a thick part of the conveyance belt 50 is in contact with the driving roller 51, as shown in FIG. 1, the belt driving effective radius r at the driving position is increased so that the belt conveyance speed is increased. On the other hand, when a thin part of the conveyance belt 50 is in contact with the driving roller 51, the belt conveyance speed is decreased.
FIG. 4 is a graph showing a belt thickness displacement at the idler roller 52 and a belt speed variation detected at the idler roller 52 when the conveyance belt 50 is conveyed at a certain speed. Even if the conveyance belt 50 is conveyed ideally without any speed variation, in a case where a thick part of the belt is in contact with the idler roller 52, the dependent effective radius “r” is increased so that the angular speed of the idler roller 52 is reduced. This is detected as a reduction of the belt conveyance speed. Furthermore, in a case where a thin part of the belt is in contact with the idler roller 52, the angular speed of the idler roller is increased and this is detected as an increase of the belt conveyance speed.
Thus, in a case where the thickness of the belt is changed, if the belt conveyance speed is detected by the angular displacement of the idler roller by using an encoder or the like, an error detection element is generated. Because of this, even if the belt is conveyed at a certain speed, due to the belt thickness displacement, the conveyance belt is detected as through the conveyance speed is changed, by the angular displacement detection of the idler roller (dependent shaft). In addition, in the above-mentioned dependent shaft feed back control, it is not possible to implement the control with high precision considering such a belt thickness displacement.
Japanese Laid-Open Patent Application Publication No. 2000-310897 discloses a method for solving such a problem caused by the belt thickness displacement. More specifically, Japanese Laid-Open Patent Application Publication No. 2000-310897 discloses a speed profile to compensate for a speed variation Vh that is expected to be generated due to the known thickness profile that extends over a whole periphery direction of a conveyance belt that is measured beforehand with a position detected by a belt mark as a standard when a driving roller is driven at a constant pulse rate; a driving motor control signal that is a modulated pulse rate against the speed variation is generated; and an oscillating motor is driven based on this and a conveyance belt is driven through the driving roller. Thus, a final speed Vb of the conveyance belt is made to be one without any variation in speed.
However, in the method disclosed in Japanese Laid-Open Patent Application Publication No. 2000-310897, the speed profile data requires data every control cycle. Hence, in a case where the control cycle is a short cycle, a large amount memory is required. In addition, in a case where the control cycle is a short cycle, the feed back control per se cannot obtain a sufficient effect. For example, in a case where the belt circumference length is 815 mm, the belt driving speed is 125 mm/s, and the control cycle is 1 ms, control of 6520 times per the belt going around one time (one revolution) is necessary, as the following formula indicates.815 mm/(125 mm/s×1 ms)=6520
In addition, if the belt thickness per one point is expressed by 16 bits, a memory having 100 Kb and more is necessary, as the following formula shows.6520×16 bit=104320 bit
Because of this, in a case where the above-mentioned control is performed by an actual image forming apparatus, a nonvolatile memory is required to be prepared as a memory for storing the belt thickness profile. Even if data are compressed and stored, and the data are expanded and loaded in a volatile memory at the time when the electric power is turned on, the large amount memory is still necessary. Because of this, a separate memory in addition to the memory used as a normal work area is necessary so that an undesirable large increase of the cost happens.
Furthermore, in the method discussed in Japanese Laid-Open Patent Application Publication No. 2000-310897, it is necessary to measure the thickness of the belt at the entire circumference as profile data of the thickness of the belt, and therefore the thickness is measured by a laser displacement gauge. Measurement data are input by input means such as an operation panel operated by a service person or at the time when the product is shipped. However, measuring means having high precision is required to measure the change of the thickness of several μm of the belt. In addition, data management of the measured result is complex. Furthermore, since the amount of data is large, an input error may be generated.